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2 Answers
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This site by climate scientist Frank Wentz discusses the rainfall possibilities of increased temperatures. Although warmer atmosphere contains more water thereby increasing rainfall, changes in circulation could minimize rainfall.

It's also possible that currently wet areas will get wetter and currently dry areas will get drier.

Dear Michael, Wentz only observes what the actual water content is but he doesn't have solid attribution of the changes or justified predictions for the future. After all, this experimenter isn't really qualified to do such things. A higher temperature of course means a higher absolute humidity for the same relative humidity but absolute humidity, and therefore temperature, is irrelevant because rain is sparked when relative humidity reaches 100 percent due to water/pressure/temperature conditions.
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Luboš MotlNov 30 '12 at 15:54

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All the guesses about changed circulation or increased non-uniformities are pure speculations and/or numerical artifacts of overfitted untested models. None of these claims actually follows from any proper physics analysis of the atmosphere and physics actually implies that the opposite sign is to be expected. Global warming, if any, would reduce all the non-uniformities in precipitation and other things.
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Luboš MotlNov 30 '12 at 15:56

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Incidentally, the fluctuations are so significant and the record is so short so far that we can't even observationally verify the proposition that the average relative humidity of the atmosphere should be more constant than the absolute humidity - because the former is between 0 and 100 percent while the latter may grow larger because of warming. The 1 deg C warming only represents a few percent change in the ratio of the two humidities but the precision with which the humidity trends may be extracted from the observations has an error that's larger. So no changes like that are obs. seen.
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Luboš MotlNov 30 '12 at 16:29

I believe that forecasting climate effects like rainfall is still a work in progress. I'm not inclined to dismiss Dr. Wentz,s views on the subject.
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Michael LuciukNov 30 '12 at 16:45

You may not be inclined but it's still true that you don't have any physical evidence for your claims, e.g. that "it's possible that wet will get wetter" etc. You (and Mr Wentz and dozens of others) just use the uncertainties in order to promote one particular kind of answer that is as unjustified as any other answer (and actually less justifiable than the opposite answer) because you find this answer more convenient, for certain reasons. It's called scientific dishonesty.
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Luboš MotlDec 1 '12 at 11:56

Because the statement isn't true while it is more convenient for the popular idea that there is some climate change to worry about.

In reality, irregularities and extremes in precipitation, temperatures etc. are driven primarily by temperature gradients. Because the cold poles are predicted to warm faster than the warm equatorial areas because of various feedbacks, the pole-equator temperature difference is predicted to drop. If true, this will reduce temperature gradients and make many location-dependent quantities more spatially uniform. A higher temporal uniformity is expected as well.

So if there were (significant) global warming, precipition and other processes would probably get more uniform and less extreme. The frequency of rains that are stronger than a given threshold would drop. However, the relative difference in the intensity of rain etc. would be very small because one or few Celsius degrees is a tiny change of the absolute temperature while the natural noise and fluctuations in quantities describing precipitation are of order 100 percent.

This is a textbook material. I was taught those things mainly by Richard Lindzen but e.g. his textbook on atmospheric physics is recommended:

You should mention that the work by Lindzen is highly controversial. Without such a mention, the claim This is textbook material is misleading, because it implies the exact opposite. For example, tropical cyclones are driven by sea surface temperatures, not by equator-pole gradients. Many models, such as the ones by Trenberth, do show an increase in intensity of precipitation events, and warmer climates do tend to have more intense precipitation.
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gerritFeb 12 '13 at 22:37

My statement that it is a "textbook material" means that every competent atmospheric physicist must learn this stuff before he or she may start to do actual serious research. The fact that a bunch of incompetent, corrupt, and ideologically driven pseudoscientists wants to make a fact about physics controversial doesn't change anything about that. What you write is complete nonsense from a physics viewpoint. All motion in the atmosphere and anywhere is always governed by gradients. A higher or lower but constant temperature clearly can't induce any macroscopic circulation.
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Luboš MotlDec 5 '14 at 7:16

There will always be a gradient between temperatures at the surface and the upper troposphere. The difference between surface and upper tropospheric temperatures increases if surface temperatures increase. Therefore, said gradients increase. More intense surface heating means more intense convection means more intense (tropical) storms. Your accusations of pretty much the entire field of atmospheric science is unscientific and unconvincing.
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gerritDec 5 '14 at 16:28

No, the gradients don't increase - the vertical gradients are given by the lapse rate. What increases is the height of the tropopause - the thickness of the troposphere. This increase due to "global warming" - CO2 or otherwise - is nearly negligible and the hurricane activity is proportional to a power of the height with a moderate exponent, so any impact of these vertical changes on the hurricane activity is guaranteed to be unobservable. It is you, not me, who is attacking atmospheric physics, starting from its most standard graduate textbooks (and from the 2nd law of thermodynamics).
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Luboš MotlDec 6 '14 at 6:44

I mentioned a critical thing here - textbook material of atmospheric physics, namely a commonly used graduate textbook covering all these basic physical processes and written by a famous MIT professor. You attacked it, without a glimpse of rational evidence, and even criticized that I used the word "textbook stuff" for textbook stuff, and you suggested something that every good high school student knows to be impossible - namely the suggestion that the thermal motion of atoms is converted to hurricanes. Energy lost in heat can't be turned into useful motion - 2nd law of thermodynamics!
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Luboš MotlDec 6 '14 at 6:46